1. Field of the Invention
The present invention relates to a method of controlling the diameter of a single crystal when the single crystal is pulled and grown from a raw material melt by the Czochralski method (hereinafter, referred to as the “CZ method”), and particularly to a method of controlling the diameter of a single crystal that enables the precise measurement of diameter of a growing single crystal.
2. Description of the Related Art
A single crystal silicon is a material for a silicon wafer used for a semiconductor device, and the CZ method excellent in mass production is widely adopted for the production of the single crystal silicon. During the growing of a single crystal by the CZ method, a seed crystal is dipped into a raw material melt of silicon retained in a crucible within a chamber maintained in an inert atmosphere under reduced pressure, and the dipped seed crystal is gradually pulled, thereby growing a single crystal silicon below the seed crystal.
When the single crystal is grown, the diameter of the single crystal is sequentially measured, and the diameter control that adjusts the pulling rate of the single crystal and the temperature of the raw material melt is performed so that the single crystal diameter becomes an aimed diameter. Conventionally, the approaches that measure the diameter of a single crystal include an optical system using an optical sensor such as a one-dimensional camera and two-dimensional camera and a weight system using a weight sensor such as a load cell.
In the optical system, the boundary (meniscus) between the single crystal and the raw material melt is imaged by the optical sensor, and the diameter of the single crystal is derived from the brightness distribution of the image data. This approach detects a ring-like portion (hereinafter, also referred to as a “fusion ring”) that appears along the boundary between the single crystal and the raw material melt and has the brightness higher than the surrounding thereof, and converts the ring-like portion into the diameter of the single crystal.
When a one-dimensional camera is used as an optical sensor, positions scanned in parallel to a line traversing the center of the fusion ring, i.e., the center of the single crystal, are imaged, and the diameter of the single crystal is determined from the distance between pixels where the peak of brightness appears. When a two-dimensional camera is used as an optical sensor, a half of the fusion ring is imaged, a circular arc formed by connecting pixels where the peak of brightness appears is calculated, and the center of the fusion ring is calculated from this circular arc so as to determine the diameter of the single crystal.
On the other hand, in the weight system, the weight of a single crystal is measured by a weight sensor, and the diameter of the single crystal is derived from the weight difference between the weight data and the model weight.
In the growing of a single crystal in which the diameter control by the optical system is adopted, a wire is typically used as a pulling shaft that holds a seed crystal and pulls the single crystal. While in growing a single crystal in which the diameter control by the weight system is adopted, a rod called a force bar is typically used as the pulling shaft.
In the diameter control of a single crystal, it is important to accurately measure the diameter of the growing single crystal. The reasons for this are as follows. If a measured diameter of a single crystal is misjudged to be larger than the actual diameter, the product quality is adversely influenced because the actual single crystal diameter is small, and further defective products are generated because the diameter of the single crystal does not reach the diameter of the product, which might incur an enormous loss. On the other hand, if the measured diameter of a single crystal is misjudged to be smaller than the actual diameter, the total length of the single crystal is shortened because the actual diameter of the single crystal is large, which might decrease the yield of the product.
However, in case of the diameter measurement by the optical system described above, since the fusion ring at the growth interface is measured and thus the response to an instantaneous diameter change is excellent, the spread of a microscopic diameter fluctuation at the measured site can be accurately detected. However, the distance from the optical sensor to the fusion ring to be measured may fluctuate due to the effects such as the strain of the wire (pulling shaft) in association with progression of the single crystal growth, and issues related to materials (quartz crucible, graphite crucible, and polysilicon) like dimensional accuracy, degradation and weight change. Therefore, in the diameter measurement by the optical system, it is difficult to detect the error of the diameter fluctuation occurring in the direction of the crystal growth, resulting in the lack of reliability in accuracy of the measured diameter of the single crystal.
On the other hand, in case of the diameter measurement by the weight system as described above, since the weight of a grown single crystal itself is measured, the change in the diameter fluctuation in the crystal growth direction in association with progression of single crystal growth is accurately detected. However, since it requires a processing for excluding the influence on the diameter fluctuation which is attributable to the buoyancy acting on the growing single crystal, its responsiveness is poor. Hence, it is difficult to detect the spread of the microscopic diameter fluctuation, resulting in the lack of reliability in accuracy of the measured diameter of the single crystal.
Accordingly, in the diameter control of a single crystal, an approach is desired that can accurately measure the diameter of a growing single crystal.
For example, Japanese Patent Application Publication No. 2003-176199 discloses a method of controlling the diameter of a single crystal in which, at the stage of growing a single crystal up until a cylindrical body portion, the diameter control is performed by the diameter measurement by the optical system, and at the stage of forming a tail portion subsequent to the body portion, in which the diameter of the single crystal is successively decreased, the diameter control is switched and performed by the diameter measurement by the weight system. In the method disclosed in this document, since the fusion ring is hidden by the body portion and cannot be sufficiently imaged at the stage of forming the tail portion, the weight system is adopted instead of the optical system so as to perform the diameter control of the single crystal.
However, also in the method of controlling the diameter of a single crystal disclosed in Japanese Patent Application Publication No. 2003-176199, the fact remains that the diameter measurement is carried out by either the optical system or the weight system at individual growth stages respectively, and therefore the diameter of a growing single crystal cannot be accurately measured.